/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        return _mergeTwoLists(list1,list2);
    }
    ListNode* _mergeTwoLists(ListNode* l1,ListNode* l2)
    {
        if(l1==nullptr)return l2;
        if(l2==nullptr)return l1;
        if(l1==nullptr&&l2==nullptr)return nullptr;

        if(l1->val<=l2->val)
        {
            l1->next=_mergeTwoLists(l1->next,l2);
            return l1;
        }
        else
        {
            l2->next=_mergeTwoLists(l1,l2->next);
            return l2;
        }
    }
};
